whereas he used the term virulence to describe the severity of the disorder induced.
Phytotoxins that affect plants are usually further categorized by the target range, from the host's range to a wider radius outside the host plant. These are technically called host selective toxins (HSTs) and non‐host selective toxins (NHSTs) [5], where HSTs affect the plant producing the phytotoxin as it hosts the endophyte while NHSTs have no effects on the host plant but affect other plants around it. The mechanisms of action of most HSTs have elements of pathogenicity, where the endophyte liable for a particular phytotoxin invades host tissues, causing some diseases in the plant. In contrast, it is not clear about the roles of NHSTs in pathogenicity [5–7]. Some authors have suggested that NHSTs are not the only pathogenicity determinants in the plants that they affect and they largely contribute to the virulence of the pathogens that produce the toxins [8]. There is a hypothesis that the compounds that a plant synthesizes resemble those that are formed from metabolic processes of endophytes within its cells [9].
2.1.2 Secondary Metabolites
Plants are known to produce various compounds, some of which are directly beneficial to human and animal health. These compounds are secondary metabolites, synthesized by plants and sometimes associated by endophytic activities. Some secondary metabolites are reported to have allelopathic activities [10] and adversely act on the host plant itself or other surrounding plants.
2.2 Possible Categorization
Depending on convenience and the details available about the phytotoxins under consideration, there are a number of ways of systematically categorizing them into meaningful classes. These could be any from: the biological characteristics, chemical characteristics, occurrence, pharmacological characteristics, taxonomic details, response to growth conditions, or habitat. However, some of these aspects can be carefully combined to better qualify other descriptions. Below are brief discussions on the possible categories that can be used to classify phytotoxins.
2.2.1 Biological Characteristics
Phytotoxins differ in terms of their biological characteristics. These characteristics can emanate from the biology of the responsible endophyte or from the bioactivities that the toxins are reported to exhibit, if already studied. Categorization of phytotoxins based on biological characteristics has been a traditional method of phytotoxin classification [8]. For instance, mycotoxins, secondary metabolites from fungi that adversely affect plants and humans alike, often fatally [11], present a good example of classification based on biological properties. The categorization of mycotoxins and other phytotoxins into two different groups had to be revised. They were made into one group of endophytic phytotoxins after similar biological activities of the toxins, among other characteristics, were noticed [11].
Some wild mushrooms, for example, have phytotoxins such as muscarine and muscimol. When ingested, these toxins cause nausea, confusion, diarrhea, visual challenges, hallucinations, and salivation. Symptoms are key to determining the effects of poisoning and, when their onset is delayed, there is only a very small chance of survival because of delayed intervention.
In the same way, some taxonomic details of the host plant can also be of great significance in categorizing toxins from their respective plants of origin. Chemotaxonomy has already proven to be reliable when markers, such as race, genera, species, and pathotypes, among others, are used to determine the phylogenetic relationships among endophytes responsible for the production of some toxic metabolites with similar traits [1].
2.2.2 Chemical Characteristics
Chemical characteristics are one of the easiest and most convenient ways to classify phytotoxins. This is usually done by examining the chemical structures of the phytotoxins, among other chemical properties. Since chemical structures are easily classified by their biosynthetic pathways, those for phytotoxins can also be done in a similar fashion. For example, phytotoxins from Alternaria and Fusarium pathogens share similar chemical structures [11]. Usually, chemical structures are categorized as polyketides, alkaloids, ribosomal and non‐ribosomal, terpenes producing peptides, and metabolites of mixed biosynthetic origin. Other examples include Stagonospora nodorum and Pyrenophora tritici‐repentis; these are both fungi and are known to produce phytotoxic metabolites that are ribosome‐produced peptides [12].
2.2.2.1 Cyanogenic Glycosides
Cyanogenic glycosides are plant‐based phytotoxins occurring in more 2000 plant species, and some of these species are used as food. Plant foods that contain these phytotoxins include sorghum, cassava, bamboo sticks, almonds, and summer fruits. Their toxicity to humans emanates from the cyanide that they contain; the clinical signs of contamination in humans include elevated breathing rates, dizziness, diarrhea, a reduction in blood pressure, headache, stomachache, nausea, confusion, convulsions, and cyanosis. If the body fails to detoxify these cyanides, it can end up in fatality.
2.2.2.2 Furocoumarins
Furocoumarins are found in many plant species, such as celery, citrus plants, and parsnips. In plants, they are released in the presence of stressful events, such as drastic climatic changes and physical damage. They may cause skin reactions and, in susceptible people, gastrointestinal problems may follow.
2.2.2.3 Lectins
Lectins are common phytotoxins in beans, with the highest concentration in red kidney beans. This is the reason why even a few beans eaten raw can cause nausea, severe stomachache, and sometimes diarrhea. At high temperatures, lectins are destroyed. In local settings, boiling beans for over 10 minutes helps to eliminate these phytotoxins.
2.2.2.4 Solanines and Chalcones
Solanines and chalcones are glycoalkaloid phytotoxins present mostly in the Solanaceae family. This is a family that includes potatoes, tomatoes, and eggplants. Generally, the concentration of these toxins is low but they are found to be high in some plants depending on the stage of growth and the environment. Unripe tomatoes and potato sprouts have relatively high levels of solanines and chalcones. Stressful conditions such as ultraviolet light, high temperatures, biochemical attack from microorganisms, and physical damage such as bruising provide a good environment for the release of these toxins.
2.2.2.5 Pyrrolizidine Alkaloids
Pyrrolizidine alkaloids are a group of toxins produced by about 600 plant species. They are most commonly found in the Asteraceae, Boraginaceae, and Fabaceae families. They generally grow as weeds that, in the course of growth, tend to contaminate food plants. Pyrrolizidine alkaloids have high stability during processing and they have been detected in honey, herbs and spices, herbal teas, and wheat products. The overall risk to health is yet to be explored [13]. This demonstrates the convenience and importance associated with the chemical characteristic classification of phytotoxins.
2.2.2.6 Pharmacological Characteristics
These are related to biological characterization, but with the addition of relevance in therapeutic, pharmacokinetic, and pharmacodynamics activities. Although endophytes usually cause diseases to the host plants, they sometimes have beneficial effects to the plants as well as the animal kingdom that consume the attacked plants. Endophytic fungi has been known to have damaging properties on the leaves of c. Papaya for example but some toxic compounds isolated from these endophytes possess very good cytotoxic properties which can be used
